Thermionic emissive cathode with end structure for emissive suppression



FOREMAN ETA March. 7, 1967 R. J. L 3,308,329

. THERMTONTC EMTssTvE cATHoDE WITH END STRUCTURE EUR EMIssTvE SUPPRESSION T l Filed Nov. 23, 1962 5m m y m R WNS o .4. m A RJY B United States Patent O 3.368,329 THERMIQNEC EMlSSlVE CATHODE WITH END STRUCTURE FUR EMlSSlVE SUPPRESSION Robert J. Foreman andrloseph A. Smith, Williamsport,

Pa., assignors, by mesne assignments, to Litton Industries, Inc., Beverly Hills, Calif., a corporation of Delaware v Filed Nov. 23, 1962, Ser. No. 239,453 2 Claims. (Cl. 313-107) This invention relates to barium-containing cathode structures used in evacuated thermionic electron emissive devices, and more particularly to a cathode structure for use in a microwave device such as a magnetron wherein discrete areas of the cathode supporting structureV are covered with a layer of alloyed material having both emission suppression and gettering properties.

In evacuated electron emissive devices it is important to have the electron emission confined to the intended cathode area in order thati proper functioning of the devices can be consistently maintained. It is `a common practice in certain of these devices such .as magnetrons to incorporate annular flanges transversely positioned one on either end of the cathode emitting surface to serve as end shields to block or prevent longitudinal electron escape from the cathode-anode interaction space into the end spaces flanking the anode. The internal geometry of the tube structure is such that the peripheral edge of each end shield is proximal to a'magnet pole piece.

Dining both the standby and operating states of tube operation there is migration of electron emitting materials from the cathode emitting surface onto the adjacent end shields thereby creating areas of spurious emission. Upon alteration of the operational status of the tube such as when switching from standby to full operation or the changing of pulse lengths or duty cycle, the areas of spurious emission on the end shields create periods of undesirable operational instability.

Since there -is a closeness of end shield and pole piece spacing and vastly different operational electrical potentials existent therebetween, spurious emission in that regi-on acting in conjunction with the magnetic and electric fields of the device constitutes an electron beam extending between the end shield and the neighboring pole piece. This detrimental condition, known in the art as high energy hollow beams, is capable of inducing arcing and causing severe erosion of the steel pole piece with the resultant produc-tion of sputtered metal and deleterious gases.` These in turn aggravatean erratic arcing situation and bring about a reduction in cavity Q which further lowers the operational efficiency andreliab-ility of the tube.

To overcome spurious emission and its consequences, a number of materials and coatings for theend shields and contiguous structures have been tried, but the disadvantages very often outweighed the intended advantages. One of these remedial measures was to employ end shields fabricated from a pure zirconium metal which exhibits well-known properties of emission suppressi-on and gettering. Pure zirconium was found to be unsatisfactory since it undergoes a transformation of crystalline structure (phase change) at a tempera-ture lower than the normal tube processing and operating temperatures. Consequently, end shields formed of zirconium displayed severe structural distortion. was applied alone as coating, there was a noticeable lack of satisfactory adherence. The shedding of the loose zirconium particles on handling made unalloyed zirconium both undesirable and unacceptable for this particular usage. yMoreover, the porous sintered layer of ir'conium exhibited inferior emission suppression capability.

When unalloyed zirconium 3,398,329 Patented Mar. 7, 1967 Accordingly, it is an object of this invention to reduce the aforementioned disadvantages by providing a cathode struc-ture that has defined areas covered with an alloy material which advantageously presents `a smooth nonporous surface which evidences no noticeable damage when subjected to elevated temperatures.

It is a further object to provide a layer of alloy material that has adequate cover-age .depth and adherence capabilities and which can` be handled readily without encountering surface damage. y

It is an additional object to provide a layer of alloy material that exhibits both emission suppression and gettering properties.

The foregoing objects are achieved in one aspect of r the invention by vacuum firing a layer of zirconium-nickel mixture to discrete areas of the end shields and the cathode supporting structure. The alloy material so formed demonstrates excellent oxide reduction and gettering properties, exhibits good cover-age of the areas concerned, surface alloys well with the refractory metal base material, and undergoes no serious degree of distortion at tube processing and operating temperatures.

' For a better understanding of the present invention, together with other and further objects, advantages, and capabilities thereof, reference is made to the following disclosure and appended claimsin connection with the accompanying drawing in which an enlarged plan View shows a portion of a cathode structure and related tube parts.

Referring to the figure, vcathode assembly structure-'11 consists of a hollow cylindrical base or spindle member 13 fabricated from a refractory metal such as molybdenum which is capable of withstanding high operational temperatures. This-cathode assembly structure 11 is designed for orientation in spaced relationship with the anode structure 15 and steel magnet pole pieces 17 and 17 which have therein longitu-dinal bores 19 and 19 to facilitate the mounting of assembly structure 11. A sleeve-type dispenser -cathode 21 composed of a tungsten matrix impregnated with an electron emitting composition such as barium aluminate is positioned on spindle member 13 in spaced relationship with anode structure 1S. An insulated tungsten heater 23 -of conventional design is suitably mounted within cathode assembly structure 11 in heat relationship with cathode 21'to facilitate thermionic electron lemission therefrom. An annular flange or end shield 25, likewise of molybdenum or tungsten, is integrally formed on spindle member-13 to provide a barrier to electrons attempting to leave the inter! action space 48 and-enter the adjacent end space S0.` Any electrons outside the interaction space constitute waste current which -reduces tube eliiciency.

A similar protective construction is embodied vin another annular flange or end shield 27, similar to flange 25, vbutintegrally attached to a molybdenum sleeve 29. This end shield sleeve 29 has an internal diameter slightly larger than the external terminal diameter of spindle 13 enabling a slidable lit thereon to abut the cathode 21, whereupon sleeve 29 is fastened'to spindle member 13 as by welding, brazing, or a similar means of securement. This second annular ang'e or end shield 27 functionsy similarly to previously described shield 25 in that it -acts as a barrier to electrons attempting to enter end space 52. vA coating of zirconium-nickel mixture is discretely applied to end shield sleeve 29 and vacuum fired prior to the mounting of sleeve 29 on spindle 13. While this process will be fully described later, it is appropriate to mention that during vacuum firing, when temperatures between 150017G0 C. are encountered, the Zr-Nil mixture melts, wets, and alloys with the molybdenum sleeve and forms thereon a smooth, nonporous surface layer of `zirconium alloy 31. During the firing process excess-nickel evaporates thereby raising the melting point v of the resultant zirconium alloy layer to temperatures well above 1200 C.; the highest temperature to which the cathode assembly structure will be subjected to either during tube processing or subsequent operation.

4In considering the completed cathode assembly structure 11, the layer of zirconium alloy Bbl-'begins at a concentric-al boundary 35 on annular flange 27 which is substantially midway on the surface of flange 27 facing the cathode v21. The alloy layer 31 which progresses Over the periphery 28 of flange 27 covers the remainder of the ange and continues along the end shield sleeve 29 to a boundary 37 near the end of the sleeve. From concentric-al boundary 35 to cathode'21 there is a bare surface sleeve area 39 that is devoid of zirconium alloy. This bare surface provides a barrier intended to prevent migration of zirconium to the cathode emitting surface.

There is a like layer of zirconium alloy 40 integrally attached to a defined surface area of spindle member 13 -beginning at a concentrical boundary 41 on annular ange- 25v which is likewise substantially midway on the surface of ange-25 facing cathode 21. This alloy layer progresses over the periphery 26 of flange 25 and covers the'remainder of the flange and continues along spindle 13 to a terminal boundary 43thereon which -is substantially within bore i19 of magnet pole piece 17. It will also be noted that from concentrical boundary 41 to cathode 21 there is a bare surface spindle area 45 that is devoid of zirconium alloy similar to the bare surface sleeve area 39 mentioned above.

The significance of the zirconium-nickel alloy material may be more fully explained in the following manner. By selection, peripheries Z6 and 28 of molybdenum end shield fianges 25 and 27 respectively are spaced adjacent surfaces 18 and 18 of steel magnet pole pieces 17 and 17'. When a tube is under operating conditions, a full pulse voltage will exist between pole piece surface 18 and ange periphery 26; and the same is true between pole piece surface 18 and flange periphery 28. In addition, there is also present in this region a permanent magnetic .field of considerable magnitude. which is parallel to the electric field. As previously mentioned, if stray electrons are present in the end spaces, these two fields constitute contributory conditions for the formation of a high energy electron beam, thus making that region of the tube particularly vulnerable to detrimental arcing. During cathode activation at approximately ll70 C. and in subsequent standby and operational conditions wherein the cathode is at a temperature of 900 C. to 990 C., there is migration of barium, barium oxide, and other cathode materialsfrom the l'surface of the cathode to adjacent surfaces such as those of the neighboring end shield structure. Defined end shield and spindle areas adjacent to the cathode 21 of cathode assembly structure 11 are adequately covered with a layer of alloy` material 3:1 and 40 that has emission suppression and gettering properties. k

As previously stated,.sintering on pure zirconium lpowder isnot satisfactory since the adherence is poor and the surface emission suppression capability is inferior. Moreover, end shields of pure solid zirconium undergo a transformation of crystalline structure ata temperature of 862 C. which causes severe surface deterioration. It has been found that by alloying another metal such as nickel with zirconium, no serious surface destruction is observed.

In the preferred embodiment, zirconium, because of its emission suppression and gettering properties, is desired as the predominant metal in the alloy. When combined with nickel, the resulting alloy exhibits improved qualities of surface coverage and bonding. It has been found that when the Zr-Ni mixture is applied and alloyed to the molybdenum base members 13 and 29, the surface of the molybdenum diffuses into the zirconium and` nickel alloy to provide an interface ternary Zr-Ni-Mo alloy. While information as to the exact conditions required to form the Zr-Ni-Mo interface combination are not fully known, it is known that the desired alloying results can be accomplished within the temperature range of l500 C. to 1700 C., depending upon the` Zr-Ni mixture employed. By increasing the weight'percentage of nickel in the Zr-Ni mixture towards the eutectic composition (17%), the temperatures required for alloy formation can be reduced to facilitate rapid alloying for various designs of cathode spindle `member 13 within temperatures safe for the structures involved.'

To achieve the desired Zr-Ni-Mo alloying, it has been found that the proportion of zirconium in the Zr-Ni mixture may be varied within the range of 83 percent to 96 percent by weight. This weight percentage range for the Zr-Ni mixture represents the proportions wherein the most advantageous temperatureV alloying results can be achieved. The terminal percentage values are not critical limits defining success or failure; they are representative values above and below which results of lesser desirability will be encountered. It is desired to haveas high a zirconium content as possible; but as the amount of zirconium is increased, the required alloying temperature also rises and inferior surface coverage is evidenced. Experience h-as shown that proportional Variations within the aforementioned range have yielded desired results. The eutectic temperature for theZr-Ni composition by weight of 83 Vpercent zirconium and 17 percent nickel is 961 C. At the upper end of the preferred compositional range, if the zirconium content is increased above 96 percent, the gradient of the alloying temperature rises to-a value above 1600" C. which approaches' the critical temperature for embrittlement of the molybdenum base members which in some applications may beundesirable. The zirconium contentof the Zr-Ni-Mo alloy exhibits the inherent reducing properties of. zirconium. The barium and barium oxide migrating from the cathode 21 comes in contact with the Zr-Ni-Mo material. The barium oxide rapidly undergoes a reduction reaction wherein zirconium oxide is formed and barium is released and evaporated from they Zr-Ni-Mo surface. The zirconium `oxide thus formed is a very st-able compound at the highestvtemperatures encountered during tube activation and operation. Thus the elimination of the spurious electron emission from migratoryemitting deposits prevents the formation of the high energy hollow beam previously described, and tube operational stabilityand efficiency are greatly improved.

Zirconium is also prominant as an efiicient high` temperature gettering element and, like all chemical gettering materials, tends to have diminishing ability due to the formation of oxides, nitrides, etc. on its surface. As a result, after tube activation and exhaust processing temperatures of ll C. to ll70 C., its surface gettering ability is greatly reduced. However, since there is an abundance of subsurface alloyed zirconium on the end yshield iianges 25 and 27 adjacent'magnet pole surfaces 18 and 18', where arcin'g in magnetrons generally occurs, the occurrence of an arc disrupts the surface compounds on the alloy material 31 and 40 and exposes a new surface of active zirconium underneath. Arcing vaporizes a small amount of metallic zirconium which combines not only with gases released during the arc but also with the gas which was responsible for initiating the arc originally. Thus a regenerative gettering or pumping'action is constantly available within the tube as the need may manifest itself during continued operation. This reserve gettering ability has the resultant benefit of maintaining desirable gas levels within the tube, whereby ylife expectancy of high power magnetrons is appreciably extended. i

An added benefit of the alloy layer is the surface char-` acteristic thereof which, being somewhat darkened, has,H

improved heat radiating qualities. vThis factor is advantageous in high power high frequency magnetrondevices to aid inthe dissipation o f `heat derived from back bombardment of the cathode.` 4The process-.for applying and alloying lthe Zr-'Ni mixturet'o the molybdenum end shield sleeve 29 and the molybdenum spindle member'13 is accomplished on each respective part before the"assembly 4of the combination.

The procedurev begins` with'the constitution of the Zr- Ni material. kkBoth 'the "zirconium Aand nickel powders comprising the mixture'have'particle `sizesranging from two to four microns. They are mixed dry only in small batches of four o rrvewgramsl sincezirconium is highly reactive to oxygenand therefore susceptible to spontaneous` combustion. Asa 'safety precaution, wet mixing is recommended if larger size batches -are considered. As a -safety expedient, zirconium 'hydride powder may be employed in place of the pure zirconium powder in the mixture.

The mixture of zirconium and nickel powders are sus-f pended in a volatile organic binder such as nitrocellulose la-cquer. A solvent such as for example amyl acetate is added to thin the mixture to a consistency proper for application to achieve desired coverage and adherence. The volatile nature of the amyl acetate makes for variable liquid consistency; but it has been found that a suspension viscosity of approximately 100 cps. is quite satisfactory for facile application of the mixture to the base members. If the suspension is too viscous, peeling may result upon the application of heat during vacuum firing.

The method for applying the Zr-Ni mixture is not cvritical as long as the desired area of the base member is adequately covered with a layer of Zr-Ni material of consistent thickness. Experimentation has shown that -a finished alloy thickness of two mils is considered a minimum requirement to insure adequate coverage. The desired thickness for satisfactory -coverage may be achieved by a build-up of two or more individually processed layers. There is no denite maximum thickness since complete surface coverage fulfills the essential requirements. Internal spacings and design dimensions must be considered accordingly. The application technique usually ernployed involves a means for rotating the base member parts while the Zr-Ni suspension is applied by ine jet air spray or brush. The rotation for application of the Zr- Ni metal to the respective individual parts is facilitated bv utilizing the chuck of a lathe for grasping end portion 47 of shield sleeve 29 and shank portion 49 of spindle member 13.

In the alloying process, the shield sleeve 29 is temporarily placed on the terminal end of the spindle member 13 and the two coated members are temperature processed simultaneously. This alloying is executed in a vacuum furnace at a pressure not to exceed 5x10*5 mm. Hg.

The actual alloying is rapidly accomplished during a ten second interval at a temperature within the range of 1500 C. to 1700 C. The exact temperature is dependent upon the proportional composition of the Zr-Ni mixture that is to be alloyed with the molybdenum surfaces to which the suspension has been applied. Since recrystallization of the molybdenum follows a time-temperature relationship, it is desirable to accomplish rapid alloying of the Zr-Ni layer to the molybdenum surface before the whole of the molybdenum structure can be subjected to the time-temperature criticality for embrittlement.

As heretofore mentioned, the weight percentages -of zirconium and nickel in the alloy can be varied within the aforementioned range to facilitate alloying with respect to the size and mass of the cathode assembly structure concerned without materially lessening the beneiits of the zirconium content. This permits a variation of the procstant.

During the high temperature vacuum firing, of short time duration, there is a chemical breakdown of the Zr-Ni suspension whereupon `theamyl acetate and nitrocellulose binder are volatilized. A portion of the nickel content is yaporized from the surface -of the alloy. While the exact nickel content of the final alloyed layer is not definitely known,v it is evidenced from processing that the higher the nickel 'content of the alloy the greater the amount of nickel vaporization. Due to the short. duration of high temperature processing, it is believed that most` of the vaporized nickel comes from the outer moleculai layers of the alloy surface.

While nickel has been described as the preferred metal to be added to the zirconium, several other finely divided or powdered transition metals can be similarly used in proper proportional combinations with zirconium under suitable temperature conditions. For this -application the material to be successfully alloyed with zirconium should be a metal having a favorable vapor pressure at the operating temperature of the cathode and one of which a minor quantity will effect a resultant reduction in the melting point of zirconium to within the range of 1500 C. to 1700 C. In addition to nickel, suitable transition metals would include cobalt, vanadium, titanium, chromium, manganese, molybdenum, and tungsten. Some examples of zirconium alloy weight combinations are:

There has thus been described a cathode structure adapted for use in a magnetron structure whereon there has been provided a layer of alloy material having reduction and gettering properties. This material is integrally alloyed with specific areas associated with the cathode assembly structure and exhibits excellent adherence, adequate coverage, and evidences excellent emission suppression and gettering ability. This results in a vast improvement in stability of tube performance and lengthening of `operational life. It has been found that leakage current and gas content within the tube have materially decreased during operational usage thereby achieving substantial gains in overall tube performance and eiciency. Tubes which have been subjected to a reaging process, necessitated by operational abuse or arcing, have successfully completed a considerable degree of extended life.

While not shown, the essentials of this invention are equally adaptable to usage -in traveling wave tubes, backward wave oscillators, klystrons, and similar devices wherein it is desirable to have discrete areas of high temperature operating electrodes exhibit emission suppression and gettering properties to enhance operational efficiency. The desired alloy mixture is applied as a coating to specific regions of the refractory metal electrode structures and Vacuum iired for alloying as previously described.

While there has been shown and described what is at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modiiications may be made therein without departing from the scope of the invention a-s delined by the appended claims.

What is claimed is:

1. A cathode assembly structure adapted for use in an evacuated high temperature thermionic electron emissive device having an anode and oppositely spaced pole pieces with axially aligned longitudinal bores comprising:

a tubular spindle axially positioned and spaced to eX- tend through said anode and into the longitudinal bores of said pole pieces,

a thermionic cathode, employing barium compounds capable of providing electron emission, mounted on said spindle in spaced relationship With said anode,

transversely outstanding annular anges functioning as emission end shields mounted on said spindle and spaced from each end of said cathode and adjacent the openings of said longitudinal bores, and

a continuous layer of a zirconium-nickel alloy having emission suppression and gettering properties integral with and extending along at least a portion of said flanges and said spindle to a position substantially within said bores.

2. A cathode assembly structure adapted for use in an evacuated high temperature thermionic electron emissive device having an anode and oppositely spaced pole pieces w-ith axially aligned longitudinal bores comprising:

a tubular spindle axially positioned and spaced to eX- tend through said anode and into the longitudinal bores of said pole pieces,

a thermionic cathode, employing barium compounds capable of providing electron emission, mounted on said spindle vin spaced relationship with said anode,

transversely outstanding annular flanges functioning as vemission end shields ,mounted on said spindle and spaced from each end of said cathode and adjacent the openingsrofk said longitudinal bores, and

a continuous layer of zirconium-nickel alloy of which zirconium comprises more than 83 percent by Weight, bonded to lat least a portion of said anges and eX- tending along said spindle providing emission suppression and gettering properties thereon.

References Cited by the Examiner UNITED STATES PATENTS 2,491,284 12/1949 l Sears u 313-178 X 2,647,216 7/1953 Brown 313-157 2,957,100 10/1960 Espersen et al. 313-346 20 JAMES W. LAWRENCE, Primary Examiner.

GEORGE N. WESTBY, Examiner.

P. C. DEMEO, Assistant Examiner. 

1. A CATHODE ASSEMBLY STRUCTURE ADAPTED FOR USE IN AN EVACUATED HIGH TEMPERATURE THERMIONIC ELECTRON EMISSIVE DEVICE HAVING AN ANODE AND OPPOSITELY SPACED POLE PIECES WITH AXIALLY ALIGNED LONGITUDINAL BORES COMPRISING: A TUBULAR SPINDLE AXIALLY POSITIONED AND SPACED TO EXTEND THROUGH SAID ANODE AND INTO THE LONGITUDINAL BORES OF SAID POLE PIECES, A THERMIONIC CATHODE, EMPLOYING BARIUM COMPOUNDS CAPABLE OF PROVIDING ELECTRON EMISSION, MOUNTED ON SAID SPINDLE IN SPACED RELATIONSHIP WITH SAID ANODE, TRANSVERSELY OUTSTANDING ANNULAR FLANGES FUNCTIONING AS EMISSION END SHIELDS MOUNTED ON SAID SPINDLE AND SPACED FROM EACH END OF SAID CATHODE AND ADJACENT THE OPENINGS F SAID LONGITUDINAL ORES, AND A CONTINUOUS LAYER OF A ZIRCONIUM-NICKEL ALLOY HAVING EMISSION SUPPRESSION AND GETTERING PROPERTIES INTEGRAL WITH AND EXTENDING ALONG AT LEAST A PORTION OF SAID FLANGES AND SAID SPINDLE TO A POSITION SUBSTANTIALLY WITHIN SAID BORES. 